Low loss, silicon nitride (SiN) based integrated waveguides have for example recently emerged as attractive platform for integrated photonic devices, such as notch filters and frequency comb generators. The combination of the large bandgap and wide transparency range with CMOS compatible fabrication make SiN highly interesting especially for nonlinear optics. Using ring resonators fabricated from high confinement, anomalous dispersion SiN waveguides, broadband frequency comb generation was achieved. However the reliable fabrication of such SiN waveguides with thickness in excess of 0.7 μm remains challenging.
Integrated silicon nitride waveguides and resonator structures are an attractive platform for nonlinear optics. SiN waveguides combine the material's large bandgap and wide transparency range with CMOS compatible fabrication and a large effective nonlinearity. Upon launching a femtosecond laser pulse inside a SiN waveguide the high effective nonlinearity leads to efficient supercontinuum generation. Moreover the fabrication of high-Q SiN microresonators with anomalous group velocity dispersion, has allowed to observe parametric oscillations in integrated SiN microresonators. Planar SiN based microresonators can thus serve as integrated frequency comb generators, via the Kerr frequency comb generation mechanism first reported in 2007.
Following this pioneering work, advances in SiN nonlinear photonics have included in recent years octave spanning frequency comb generation, the observation of phase locked states (via sub-comb synchronization), and recently, the demonstration of dissipative temporal solitons and soliton induced Cherenkov radiation in SiN microresonators. SiN microresonator frequency combs have a high application potential and several promising applications have been demonstrated such as the use of low phase noise SiN comb states in coherent communication with Tb/s datarates and the observation of ultrafast optical waveform generation.
So far integrated waveguides, based on SiN or other materials, are typically fabricated using a subtractive process approach: the waveguide structures are etched into a previously deposited thin film of the waveguide material. Due to the dispersion properties, high confinement waveguides with heights in excess of 0.7 μm are required for efficient nonlinear processes. FIG. 1 shows the problems that arise during the fabrication of such high confinement SiN waveguides. SiN thin films typically exhibit high tensile stress and tend to crack when deposited thicker than 250 nm (FIG. 1a). Such cracks cause high scattering losses in the waveguide and must be avoided. Further the gap between two closely spaced, thick waveguides can present a critical aspect ratio for several processing steps. The dry etch process is typically optimized to produce smooth waveguide sidewalls, to limit scattering losses, but often does not accurately transfer a resist mask pattern for narrow aspect ratios. Due to this etch limitations slanted sidewalls and locally different etch rates are often a problem.
The SiN etch process is a low power etch process optimized for smooth sidewalls, that can however create only aspect ratios of 2:1 (FIG. 1b). During the consequent SiO2 cladding deposition, voids can form in-between closely spaced waveguides due to the limited conformality of the deposition processes (FIG. 1b).
It was also found that silicon oxide layers form between the SiN layers when using multistep deposition with thermal cycling to deposit the SiN thin films (FIG. 1c). The effect of these oxide layers on the optical performance remains unclear.